In the improbable surroundings of the South China Sea, hostilities between Liverpool and Manchester United are threatening to spill over as their respective pre-season visits to Asia are overshadowed by accusations of an illegal approach in the Merseyside club’s bid to sign Gabriel Heinze.

While the Argentina full back’s lawyers explore the possibility of legal action against United, citing a document that they feel entitles him to join any club offering €10 million [about £6.8 million] for his services, Sir Alex Ferguson hinted last night that an illegal approach may have been made. While he believes that the chief culprits are Heinze’s agents, United’s displeasure with Liverpool is clear.

“Gabriel Heinze will not be joining Liverpool as far as I’m concerned,” Ferguson said after United’s 6-0 victory over Shenzen FC in Macau, 40 miles from Hong Kong, where Liverpool are contesting the Premier League Asia Cup. “We are examining some of the statements coming from his agent and when contact was first made with Liverpool. We are not happy with the agent’s conduct in the matter and we are examining that, but it will take a couple of days before we have anything more to say.”

No senior player has moved between the clubs since Phil Chisnall in 1964 and though Liverpool and United have a cordial relationship at boardroom level, the saga is causing ill feeling. Liverpool believe that Ferguson is powerless to prevent Heinze moving because of the letter that authorised his representatives to find a new club, while United, who claim that the letter is “meaningless”, are unhappy that their rivals refuse to drop the matter despite having a £6.8 million bid rejected last week.

Heinze, who is on holiday after playing for Argentina in the Copa America, has yet to make his intentions clear, but his agent, Roberto Rodriguez, and his brother, Sebastian, who acts as his adviser, have infuriated United with their conduct. Rodriguez, who is based in Madrid, could not be contacted last night to answer Ferguson’s claims. An Argentina-based colleague at the Soccer SA agency said: “We know nothing about what is happening with Gabriel.”

David Gill, the United chief executive, said last week that he would refuse to sell any player to Liverpool, Arsenal or Chelsea, but Rafael Benítez, the Liverpool manager, is undeterred. “All I can say is we’ve made an offer, they rejected it and now [Heinze’s] lawyers are working,” Benítez said. “Now we must wait. He’s a player we like and he’s a very good player. That’s the reason why they don’t want him to leave for us.”

Even without Heinze and numerous others – including Paul Scholes, who has flown back to Britain to undergo exploratory surgery on a knee problem that is likely to rule him out of the opening weeks of the season – United were far too strong for Shenzen yesterday. Nani, their new signing from Sporting Lisbon, scored on his first appearance in United colours, with Ryan Giggs, Wayne Rooney, John O’Shea, Cristiano Ronaldo and Chris Eagles also on the mark.

Nevertheless, there is concern over Scholes and Owen Hargreaves, who has yet to figure in three matches on tour because of a knee injury, while Gary Neville and Louis Saha are likely to miss the start of the new season.

Liverpool, who have agreed a new three-year contract with Steve Finnan, play South China in the Asia Cup today in Hong Kong, while Portsmouth take on Fulham.

— Fifa should announce today whether it will arbitrate in the row over the transfer of Carlos Tévez from West Ham United to Manchester United. The sport’s world governing body met representatives of the Premier League and the FA yesterday. Manchester United are refusing to pay a fee to West Ham because they believe that the London club do not own the Argentina forward. Tévez’s economic rights are owned by a third party, but he is registered with West Ham.